\begin{table}[htbp]\centering
\def\sym#1{\ifmmode^{#1}\else\(^{#1}\)\fi}
\caption{Marginal effects of covariates, with controls}
\begin{tabular}{l*{3}{c}}
\hline\hline
            &          US         &          UK         &     Germany         \\
            &                     &                     &                     \\
\hline
Linear time trend&    -0.00087\sym{***}&    -0.00206         &     0.00105\sym{***}\\
            &   (0.00017)         &   (0.00525)         &   (0.00021)         \\
[1em]
Unemployment rate*100&     0.01150\sym{***}&     0.01501\sym{**} &     0.01939\sym{***}\\
            &   (0.00113)         &   (0.00494)         &   (0.00081)         \\
\hline
N           &       20271         &      192029         &      181707         \\
\hline\hline
\multicolumn{4}{l}{\footnotesize Standard errors in parentheses. * p < 0.05, ** p < 0.01, *** p < 0.001}\\
\end{tabular}
\end{table}
